"If 160 million women were missing from the U.S. population, you would notice," writes Mara Hvistendahl, a Minnesota-born science writer who has lived in Shanghai for the past five years.

Hvistendahl is illustrating the consequences of sex selection, most famously practiced by parents in parts of China and India. Thanks to sex-selective abortion, female infanticide and high-tech in-vitro fertilization, an estimated 160 million women are currently missing from the planet -- that's "more than the entire female population of the United States."

Hvistendahl's first book, "Unnatural Selection," explores this troubling trend from a variety of perspectives -- from the 1960s American activists who once considered sex selection a harmless consequence of population control, to the so-called surplus men of Asia who now face a shortage of potential brides.

We reached Hvistendahl last month in Rotterdam, via Skype.

Q Tell us something of your Minnesota childhood, with its unusual ties to China.

A My mother was the daughter of a Lutheran missionary. She grew up partly in Asia. Then she moved back to the U.S., went to Carleton, studied Chinese at St. Olaf in the '70s because Carleton didn't have a program back then. She ended up becoming friends with her Chinese tutor, Hongyu. When my parents divorced, my mom and Hongyu decided to become roommates. So we lived on Nokomis Court in south Minneapolis. For several years, we were this odd little family. Later, I studied Chinese at Hopkins High School via remote access learning. There were three or four people in my class. It wasn't a very popular language back then.

Q But you didn't learn about sex selection until college. How did that come about?

A I went to Beijing in 2000 for study abroad. My class took a field trip to a local elementary school and you could see it -- really, the gender imbalance is most evident in schools, where all these children are gathered. I remember our teacher asking us during the bus ride home, "Did you notice there were more boys than girls?"

Q You write that sex selection occurs in conjunction with economic development. Why does the problem rise in proportion to wealth and social status?

A As countries develop, fertility rates tend to fall. In some cases, this is natural -- people decide to have fewer kids. In other cases, it's been spurred by coercive policies; I spend a lot of time talking about population control in the book. Whatever the case, a falling birth rate is strongly linked to sex-ratio imbalance.

Q Abortion is the primary tool of sex selection. As a pro-choice American woman, how does this complicate your views on the issue?

A I still believe in a woman's right to abortion. But I don't believe choice should extend to the sex of the fetus or other characteristics like eye color.

Q Your book warns that sex selection is spreading to countries beyond Asia. What's the most shocking example?

A From the little data we have to go on, the sex ratio looks worse in Armenia than in China. The basic trends that occur in these places are, one, fertility rates decrease and, two, abortion is legal and also widely used.

Q You also write about in-vitro fertilization in the United States, where parents tend to select for girls. Why do you suppose that's the case?

A Parents give all sorts of reasons. Some are predictable -- they want a child they can dress in pink dresses and throw princess parties for. In other cases, it might be that girls do better in school or have fewer behavioral issues. For me, I don't see a huge ethical difference between this and what's happening in Asia.

Q You're the first reporter to uncover the connections between Cold War-era American politics and sex selection in Asia. How did you make this discovery?

A I was in India interviewing people for the book. Several people suggested, "You might want to look at the role of the U.S. population control movement of the 1960s and '70s." What I found was that in the beginning of the 1950s, when population growth was an issue that many Americans were concerned about, sex selection was a strategy that several scientists and thinkers proposed as a good idea. One of the things that kept coming up was that couples wanted at least one son. So then the idea exploded -- what if we could guarantee a son the first or second time around?

Q Your subject is so upsetting -- and yet "Unnatural Selection" is surprisingly addictive with its rich characters and scenes. Did any writers inspire your style?

A I admire the way Philip Gourevitch writes about Rwanda -- you know, the book with the really long title, "We Wish to Inform You That Tomorrow We Will Be Killed With Our Families."

Q On a lighter note, what places do you like to visit when you return to the Twin Cities?